The sugar cane stalk chopping systems in most modern cane harvesters include a chopper drum arrangement comprising a pair of counter-rotating drums to which cutting knives are attached. In early versions of such chopper drum arrangements, such as that disclosed in U.S. Pat. No. 3,141,281, the blades did not overlap each other so no blade-on-blade contact occurred. This arrangement did not work well for cutting green leaves when the blades were new and, as the blades wore, they did not completely severe the cane stalks. An improvement was made later wherein the cylindrical path traced by cutting edges of the blades carried by one drum overlapped the cylindrical path traced by the cutting edges of the blades carried by the other drum. U.S. Pat. No. 4,065,912 discloses this improved type of chopping system, but the intent was for the blades carried by the separate drums to have a slight clearance between each other as they intermeshed. This arrangement also had difficulties cutting green leaves.
Over the years, most chopper drum arrangements have been constructed so that beveled surfaces of the cutting edges of the blades are either opposed to each other, i.e., arranged bevel-to-bevel, or are arranged with the beveled surface of one opposed to the backside of the other, and, in both cases, making hard contact with each other. U.S. Pat. No. 3,995,520 discloses the bevel-to-bevel arrangement, which is self-sharpening. This is necessary to cut green leaves and stalks as the beveled cutting edges of the blades wear or get chipped.
However, for a variety of reasons, more and more cane is cut green and there has been an accompanying change to what is called a differential chopper. In a differential chopper, the blades on the top drum are mounted to have larger cutting diameters than the blades of the lower drum. This accelerates the speed of the top portion of the mat of the chopped cane so that it is faster than the speed of the lower portion, resulting in the mat of cane stalks being spread out as it enters the cleaning chamber located adjacent to the chopper drums, thus resulting in improved cleaning. However, this difference in cutting diameters creates the problem of the blades not intermeshing as effectively as they do when the cutting diameters are equal. Specifically, with the different cutting diameters, the blades first contact each other as the blades enter the intermesh zone, and then lose contact with each other as they move over center and exit the intermesh zone. Thus, if the entering or initial angle of contact is small and the upper blade leads the lower blade, the upper blade will undergo self-sharpening, but the lower blade will undergo self-dulling.
What is needed is a blade design for differential chopper drums that have a small angle of contact both when entering and exiting the intermesh zone such that a self-sharpening arrangement for both blades can be had. This will increase blade change intervals thus decreasing operating costs of the machine on which they are used.